In a present mobile communication cellular system, radio communication is performed between a mobile station and a base station by using a base station specific scramble code. Assignment of the scramble code is uniquely managed by a central node that controls a plurality of base stations. A system configuration in this case is shown in FIG. 13.
A mobile communication cellular system 100 shown in FIG. 13 includes, on a not-shown network, a central node 101 having an RNC (Radio Network Controller) function. A plurality of base stations (Node B) 102, . . . , 102 are connected under the central node 101. The base stations 102, . . . , 102 perform radio communication with a plurality of mobile stations (UE: User Equipment) 103, . . . , 103, respectively, in their respective service areas (cells). A base station specific scramble code used between the base station 102 and the mobile station 103 at the communication time is managed by the RNC of the central node 101.
Non-Patent Document 1 proposes a network configuration of the mobile communication cellular system where the system can work without such a central node. A system configuration in this case is shown in FIG. 14.
A mobile communication cellular system 110 shown in FIG. 14 includes, on a not-shown network, a plurality of base stations (Node B) 111, . . . , 111 each having an RNC function. The base stations 111, . . . , 111 perform radio communication with a plurality of mobile stations (UE) 112, . . . , 112, respectively, in their respective service areas (cells). Development of such a system capable of working without the abovementioned central node on a network is now demanded.
In view of this, Patent Document 1 proposes a method in which a base station autonomously makes assignment of a scramble code without depending on the central node. The method in which the base station of Patent Document 1 autonomously makes assignment of a scramble code will be described with reference to FIGS. 15 and 16.
FIG. 15 is a flowchart showing the operation of a conventional base station.
When being activated, the base station selects a code for exclusive use at activation time (step S101). The base station then starts a communication service and, at the same time, activates a timer (step S102). Subsequently, the base station determines whether a notification concerning a scramble code has been received from a mobile station (step S103). When determining that the notification concerning a code has not received from the mobile station (NO in step S103), the base station proceeds to step S106. On the other hand, when determining that the notification concerning a code has received from the mobile station (YES in step S103), the base station stores the notification information (step S104) and notifies the mobile station of information concerning scramble codes of its surrounding base stations that the base station has stored (step S105).
Then, the base station determines whether the timer activated in step S102 has ended (step S106). When determining that the timer has not ended (NO in step S106), the base station returns to step S103. On the other hand, when determining that the timer has ended (YES in step S106), the base station uses information concerning the scramble codes of its surrounding base stations that the base station has stored to reselect a scramble code from service scramble codes (step S107).
FIG. 16 is a flowchart showing the operation of a conventional mobile station.
The mobile station identifies scramble codes of the base stations surrounding the target base station when performing a hand-off process and stores them in a scramble code management table of the mobile station (step S111). Then, the mobile station notifies the target base station of information concerning the base stations surrounding the target base station which is stored in the scramble code management table (step S112). Subsequently, the mobile station determines whether a notification of information concerning the base stations surrounding the target base station which is stored in a scramble code management table of the target base station has been received (step S113). When determining that the notification has not been received from the target base station (NO in step S113), the mobile station ends its operation. On the other hand, when determining that the notification has been received from the target base station (YES in step S113), the mobile station updates the scramble code management table (step S114) and ends its operation.    Patent Document 1: JP-A 2005-142967    Non-Patent Document 1: 3GPP, R3-051105 NTT DoCoMo, “Location of RRC&RRM functions for EUTRAN”, October 2005